Conventional X-ray computed tomography apparatuses (hereinafter referred to as X-ray CT apparatuses) are mainly comprised of third-generation X-ray CT apparatuses.
In the third-generation X-ray CT apparatus, an X-ray tube and an X-ray detector as a pair are rotated around a subject, and X-rays, which have passed through the subject, are detected by a plurality of X-ray detection elements included in the X-ray detector. Incidentally, the respective X-ray detection elements are arrayed in a grid shape in a channel direction which is perpendicular to the body axis of the subject, and in a column direction along the body axis of the subject.
Detection signals of detected analog quantities are sequentially read out to a data-acquisition module (data acquisition system (DAS)) from read-out lines associated with respective channels via switches which are changed over on a column-by-column basis. This read-out method is called a “sequential acquisition method”.
The data-acquisition module includes an amplifier and an A/D conversion module, which are provided in association with each of the plural X-ray detection elements, and a control board. The amplifier and A/D conversion module amplify the detection signal acquired from each X-ray detection element, and then convert the detection signal to digital data. The control board sends this digital data to a reconstruction module.
The reconstruction module reconstructs an image relating to the subject, based on the sent detection data.
However, in the X-ray CT apparatus as described above, data-acquisition modules, which are connected to the elements arranged near the center in the channel direction (hereinafter, also referred to as “channel center”), always acquire the same projection data from X-rays which have passed through the rotational center. Thus, if there is a variance in characteristics between the elements or the data-acquisition modules, a ring artifact of a diameter corresponding to the variance tends to occur. In particular, the variance in characteristics between the elements or the data-acquisition modules in the vicinity of the channel center tends to generate an artifact in inverse proportion to a square root of the radius from the channel center. It is thus required that the variance in characteristics between the elements and between the data-acquisition modules in the vicinity of the channel center be as small as possible.
For example, in the case of such characteristics that the linearity of the amplifier and A/D conversion module (QV/ADC) connected to the element varies from chip to chip, there is a case in which an artifact occurs due to the difference in responsivity between chips. On the other hand, it is very difficult to uniformize the characteristics of individual chips, from the standpoint of the present measurement techniques, yields, etc.
The object is to provide an X-ray computed tomography apparatus which can prevent the occurrence of an artifact corresponding to the variation in characteristics in the elements or data-acquisition systems in the vicinity of the channel center.